Soil quality — Determination of water content in the unsaturated zone — Neutron depth probe method

Specifies a non-destructive method for the determination of water content in the unsaturated zone of soils using a neutron depth probe. Water content profiles can be obtained by measuring at a series of depths of soil.

Qualité du sol — Détermination de la teneur en eau de la zone non saturée — Méthode à la sonde à neutrons de profondeur

La présente Norme internationale prescrit une méthode in situ pour la détermination de la teneur en eau de la zone non saturée des sols à l'aide d'une sonde à neutrons de profondeur. Elle est applicable lorsqu'on poursuit des investigations sur la réserve en eau, sur l'équilibre de l'eau et sur la distribution de l'eau dans la zone non saturée du sol. Cette méthode étant non destructive, elle est particulièrement adaptée à des mesurages répétés au même emplacement. Plusieurs profils de la teneur en eau peuvent être obtenus en effectuant une série de mesurages à différentes profondeurs jusqu'au niveau de la nappe phréatique. L'avantage de la méthode, comparée à d'autres, par exemple la méthode à la sonde gamma, est la rapidité à laquelle les mesurages peuvent être effectués. Elle présente cependant l'inconvénient de n'offrir qu'une résolution relativement faible des mesurages de profondeur.

Kakovost tal – Določevanje vode v nenasičeni coni – Metoda z uporabo nevtronske sonde

General Information

Status
Published
Publication Date
26-Dec-1995
Current Stage
9093 - International Standard confirmed
Start Date
13-Dec-2007
Completion Date
25-Sep-2018

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INTERNATIONAL
STANDARD
First edition
1995-12-15
Soil quality - Determination of water
content in the unsaturated zone - Neutron
depth probe method
Qualit& du so/ - Determination de Ia teneur en eau de Ia zone non saturke
- Methode a Ia Sonde ;j neutrons de profondeur
Reference number
ISO 10573: 1995(E)
---------------------- Page: 1 ----------------------
ISO 10573:1995(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide
federation of national Standards bodies (ISO member bodies). The work of
preparing International Standards is normally carried out through ISO
technical committees. Esch member body interested in a subject for
which a technical committee has been established has the right to be
represented on that committee. International organizations, governmental
and non-governmental, in liaison with ISO, also take part in the work. ISO
collaborates closely with the International Electrotechnical Commission
(IEC) on all matters of electrotechnical standardization.
Draft International Standards adopted by the technical committees are
circulated to the member bodies for voting. Publication as an International
Standard requires approval by at least 75 % of the member bodies casting
a vote.
International Standard ISO 10573 was prepared by Technical Committee
lSO/TC 190, Seil quality, Subcommittee SC 5, Physical methods.
Annexes A, B, C, D and E of this International Standard are for information
only.
0 ISO 1995

All rights reserved. Unless otherwise specified, no par-t of this publication may be

reproduced or utilized in any form or by any means, electronie or mechanical, including

photocopying and microfilm, without Permission in writing from the publisher.
International Organization for Standardization
Case Postale 56 l CH-l 211 Geneve 20 l Switzerland
Printed in Switzerland
---------------------- Page: 2 ----------------------
ISO 10573:1995(E)
INTERNATIONAL STANDARD @ ISO
Soil quality - Determination of water content in the
unsaturated Zone - Neutron depth probe method
- Neutron depth probes contain radioactive sources which will present health and
WARNING

environmental hazards if a probe is improperly used, stored or disposed of. National and international

legislation and regulations must be complied with.
vestigate the possibility of applying the most recent
1 Scope
editions of the Standards indicated below. Members
of IEC and ISO maintain registers of currently valid
This International Standard specifies an in situ method
International Standards.
for the determination of water content in the unsatu-
rated zone of soils using a neutron depth probe. lt is
ISO 11272: -l), Seil quality - Determination of dry
applicable when investigations into the water storage,
bulk density.
water balance and water distribution in the unsatu-
rated zone of the soil are carried out. Because the
ISO 11461: -l) Seil quality -
Determination of soil
method is non-destructive, it is particularly suitable for
wa ter content wlculated on a volume basis - Gravi-
repeated measurements at fixed locations. Water
metric method.
content profiles tan be obtained by measuring at a
series of depths down to any depth within the range
of the phreatic level at the site.
3 Definitions
The advantage of the method compared with some
others, for example the gamma probe method, is the
For the purposes of this International Standard, the
rapidity with which measurements tan be carried out.
following definition applies.
A disadvantage, however, is the relatively poor depth
resolution of the measurements.
3.1 water content volume fraction, 8 : The volume
of water evaporating from soil when dried to constant
mass at 105 OC, divided by the original bulk volume of
the soil.
2 Normative references
NOTES
The following Standards contain provisions which,
through reference in this text, constitute provisions of
1 The water content may be expressed as a percentage
this International Standard. At the time of publication,
by volume or a volume fraction.
the editions indicated were valid. All Standards are
subject to revision, and Parties to agreements based
2 In this International Standard, water content as defined

on this International Standard are encouraged to in- above may also be referred to as “free water ”.

1) To be published.
---------------------- Page: 3 ----------------------
ISO ‘10573:1995(E) 0 ISO

3 The procedure for drying soil to constant mass at 105 “C - the gradients in this composition that occur within the

is described in ISO 11461. measuring volume;
- the gradients in soil water content that occur within
4 The procedure for determination of the bulk volume of
the measuring volume;
soil is described in ISO 11272.
- the method of access tube installation;
- the characteristics of the access tubing;
- the specifications sf the apparatus used.
4 Principle
The calibration curve usually differs for each soil layer. In
homogeneous layers that are thicker than the measuring
A neutron depth probe, consisting of a neutron Source
volume, calibration curves are generally linear, their para-
and detector, is lowered into a vertical access tube in
meters depending on the soil composition. In the case of
the soil. The neutron Source, usually of the 241Am-Be
thin or non-homogeneous soil Iayers, however, calibration
type, emits neutrons of high kinetic energy. The neu-
curves will often be non-linear due to the different effects
trons lose part of their energy when they collide with
of gradients in soil composition and water content under
atomic nuclei. After several collisions, their energy
wet and dry conditions.
level is reduced to the thermal energy level corres-
ponding to the prevailing temperature. This level is
reached most rapidly when neutrons collide with hydro-
gen nuclei because their masses are almost equal.
5 Apparatus
The thermal neutrons form a stable cloud, the concen-

tration of which is determined by the detector in the 5.1 Neutron depth probe, consisting of a fast neu-

probe. The number of thermal neutrons registered by tron Source and a thermal neutron detector combined

the detector per unit time (the count rate) is therefore with a read-out unit.
a measure of the concentration of hydrogen nuclei in
the soil around the probe. In general, the majority of
5.2 Thin-walled access tu iing, with an inner
those nuclei are in water molecules and therefore the
diameter slightly larger than that of the neutron probe.
count rate is also a measure of the soil water content.
The tubing shall consist sf rnaterial that is very
A calibration curve is used to convert the neutron
“transparent” to fast and thermal neutrons (e.g. alu-
count rate to soil water content.
minium, aluminium alloy) and which is resistant to
Chemical corrosion and to deformation due to instal-
NOTES
lation activities. Stainless steel, galvanized iron and
plastics (polyethylene) are also suitable, thouqh less
5 The neutron count rate obtained is influenced by the .
transparent to neutrons.
presence of all the atomic nuclei in the soil. However, the
count rate at a given water content may be increased in
some soils because of the thermalization of neutrons by
5.3 Equipment for instailing access tubes.
collisions with nuclei of certain soil elements, or because
much hydrogen is present in substances other than free
water. However, the count rate may be decreased because
ent for drying and cleaning the aceess
of absorption of neutrons by nuclei with a large atomic ab-

sorption Cross-section. See annex A. tubes, if necessary, a dummy probe for testing the

tubing Performance.
6 The soil volume (measuring volume) to which the measure-
ment refers approximates a sphere. For a given type of

neutron probe, the radius of the sphere depends on the to- curwes, for conversion of count rate

tal density of atomic nuclei in the soil. For the majority of
probes used in practice, the radius of the volume from
which 95 % of the neutrons counted by the detector are

generated ( “the sphere of importante” 111) tan vary from 5.6 Usual apparatus for takin so%8 samples, for

0,l m to 0,2 m in wet soil to 0,8 m or more in dry (sandy)
carrying out a field calibration to determine the volu-
soil. Consequently, the measurement obtained at a given
metric water content 8 gravimetrically according to
depth is influenced by the water content distribution within
ISO 11461.
the measuring volume at that time, and by any other gra-
dients in soil composition. Therefore, reproduction of the
measurement of a given water content at a certain depth is
only possible when the distributions of water content and
of soil composition within the measuring volume are time-
invariant. This requirement (local time-invariant gradients) is
important for the calibration of the neutron depth probe.
6.1 Installation of access tubes
See annex A.
lters of the calibration curve de-
7 The shape and parame
The location shall be representative of the immediate
Ilowing (see [2] in a nnex E):
pend on the fo
surroundings and care shall be taken to avoid surface
water from concentrating on the Spot. Use a platform
of the soil horizon considered
- the Chemical composition
to prevent darnage to surrounding Vegetation and
and its bulk d ensity;
---------------------- Page: 4 ----------------------
0 ISO
ISO 10573:1995(E)
each soil layer in accordance with ISO 11461, under
compaction of the soil surface whilst installing a tube.

Ensure that radial soil compaction around the tube, several different hydrological conditions, to derive a

compaction below it and the creation of voids adjacent calibration curve for each layer.

to it are prevented as far as possible.
NOTE IO The subdivision of the soil Profile into layers is
determined initially by differentes in soil composition, but
Install access tubes by either of the following methods.
the form of soil water content gradients that systemati-
cally recur should also be considered. Further divisions may
a) Push the tube into the soil using a hammer and
be necessary to meet the objectives of the investigation.
empty it using an auger. lt is recommended that
the lower end of the tube be closed with quick
The hydrological conditions under which the calibra-
drying cement or a stopper, to prevent infiltration
tion is conducted shall differ as much as possible so
of ground water.
that the calibration curves are representative of the
range of conditions which occur at the site. To meet
b) Push the tube into a prepared hole of the same or
the requirement for time invariant gradients that do
slightly smaller diameter and of the required
not vary with time as much as possible, the calibration
depth, then seal the lower end as in 6.1 .l . Alterna-
shall not be conducted after heavy rain or irrigation
tively, the lower end of the tube may be sealed
applications, or immediately after the sudden begin-
before insertion.
ning of extremely warm weather.
Holes tan be prepared using a guide tube or an auger
Determine the calibration curves by analysing the va-
or by a combination of these two methods. Close the
rious combinations of neutron count rate and water
top of the tube with a tight rubber stopper to keep out
content for each soil layer by regression analysis. The
rain or surface water. The tubing shall always be dry
count rate is considered as the independent variable
inside.
(x) and the water content as the dependent variable
(y). Calibration curves so derived are specific to the
NOTES
neutron probe used. Use of reference counts to nor-
malize the count rate measurements used in the re-
8 lt is recommended that access tubes be tut to protrude
gression allows calibrations to be used with different
above the soil surface as little as the apparatus permits, so
probes of the Same geometry (see annex C).
as to minimize the radiation dose received by the Operator
when lowering the probe.
Further guidelines for carrying out a field calibration

9 More specific guidelines for installation are given in [3] are given in [Zl, [3], [4] in annex E and in annex B.

and [4] in annex E.
NOTES
After installation, take great care to minimize distur-
11 The calibration CU rves may Change in
time due to the
bance of the soil and Vegetation at the site whilst
foll owing processes:
conducting measurements in the access tube.
- changes in the Chemical composition of the soil including
that of the soil water, and changes in bulk density. This
tan be corrected for, to a certain extent, on the basis
6.2 Calibration
of known (Chemical) properties (see [3] in annex E);
- decrease of the Source strength of the probe due to ra-
In most cases, calibration curves supplied by neutron
dioactive decay, and/or decrease in the sensitivity of
probe manufacturers, and those published in the Iitera-
the detector. This tan be corrected for by the use of
ture, give only a rough indication of the absolute soil
reference counts made in a medium with invariant
water content, because no or insufficient recognition
characteristics (see annex C).
tan be given to the specific influences of the site

mentioned in note 7 in clause 4 (see also annex A). 12 The guidelines given here apply to the measurement of

absolute water content. When only relative measurements
(i.e. changes of water content in time) are to be assessed,
The influence of Chemical composition and bulk den-
the requirements for calibration and demands on accuracy
sity (see A.2) is accounted for in calibrations derived
may be less stringent.
theoretically from the macroscopic neutron-interaction
Cross-section of the soil concerned (see [ 11, [4], [9] in
annex E).
The combined influence of gradients in water content,
6.3 Measurements
Chemical composition and bulk density is only accounted

for by a field calibration. Therefore an in situ field cali- The neutron depth probe shall be used in accordance

bration is necessary for accurate measurements of with the manufacturer ’s instructions as much as poss-

absolute water content. ible, and particularly with respect to technical handling

and safety.
The field calibration is based on simultaneous deter-
Lower the probe in the ac cess-tube to the depth at
mination of the neutron count rate and sampling for
which It IS required to ma ke the meas uremen t.
the determination of the volumetric water content of
---------------------- Page: 5 ----------------------
ISO 10573:1995(E) 0 ISO

Conduct the counts according to one of the following ed by appropriately skilled persons. Periodic Checks to

methods: test for leakage from the sealed Source shall be car-
ried out by a competent agency.
with a fixed counting ti me; in this case the num-
ber of thermal neutrons detected is reco rded;
with a fixed number of detected thermal neu-
trons; in this case the counting time is recorded. 7 Expressisn of results
NOTES
Calculate the count rate R, which is the number of de-
tected thermal neutrons per unit of time, using the
13 When changes of watet- content in time are to be de-
following equation:
termined, precise positioning of the probe at a specified
depth is important.
R N
14 The second method mentioned for taking the counts has
the advantage that the accuracy of the measurement is rela-
where
tively constant (i.e. precision of the count rate), whereas the ac-
curacy depends on the water content in the first method.
R is the count rate, in counts per minute;
N is the number of counted thermal neutrons;
Instead of conducting a Single count for a long time, it
tan be advantageous to make a number of counts for
t is the counting time, in minutes.
a short time because this provides quantitative infor-
mation about the spread of the measurements. This
Calculate the water content 8, using the equation:
information allows detection of certain types sf failure
in the apparatus.
0 = f fR9 PJ
lt is recommended that reference counts in a medium
where
with invariant characteristics, such as a large water
barrel (see C.3.1), be made at frequent intervals to
0 is the water content, expressed as a volume
check the Overall Performance of the instrument. For
fraction;
example, a reference count might be carried out
is the calibration function (calibration curve)
before and after each series of measurements in a
calculated by regression analysis;
specific access tube. A certain amount of drift in the

reference count is to be expected. However, a sudden R is the count rate, in counts per minute;

Change from the general Pattern almost certainly indi-
represents the Parameters of the calibration
cates a failure of the apparatus, which should be re-
curve.
paired or replaced.
When necessary, the count rate tan be corrected for
the differente between the actual reference count
6.4 Safety and maintenance
rate (R,) and the expected reference count rate IR,,).
In most cases, a correction of the type R’ = R(R,,IR,)
SAFETY PRECAUTIONS - The radioactive Source
may apply, where R’ is the corrected count rate. For
within a neutron depth probe is a potential hazard
further explanations, see annex C.
to the Operator, the public and the environment.
Most governments and Organkations have legally
enforceable regulations concerning the acqui-
sition, Operation, transport, storage and disposal
8 Accuracy
of radioactive devices, which must be adhered to.
In the absence of specific radiological safety regu-
.I The accuracy of the water content determined
lations, the guidelines of the International Atomic
with the neutron probe is influenced principally by the
Energy Agency Kl, r7] and of the International
following error sources.
Commission on Radiological Protection [*l should
be consulted.
a) The scatter in individual counts or count times as
a result of the random Variation in the number of
The half-life (458 years) of the americium commonly
neutrons emitted by the neutron Source.
used in neutron depth probes is longer than the time
over which the integrity of the Source Container (e.g.
The magnitude of this error is usually expressed
about 30 years) tan be expected to last. When a neu-
as the Standard deviation of the number of neu-
tron depth probe is no longer required, the radioactive
trons counted. As the emission process follows a
Source must be disposed of at a repository for radio-
Poisson distribution, the resulting Standard devi-
active waste.
ation in the number of detected neutrons is
Neutro n depth probes shall only be used by suitably
SN = JN
trained 0 #perato rs. Main tenance s hall only be conduct-
---------------------- Page: 6 ----------------------
@ ISO ISO 10573:1995(E)
nex E). For soils that are more spatially variable with
The inaccuracy of the calibration curve used.
respect to water content (particularly clay, alluvium
This tan be determined from the results of the
and peat soils), a greater effort is necessary to resch
regression analysis used to derive the curve. Within
that accuracy. Further details with respect to conduc-
the field calibration, the following sources of errors
ting measurements and determination of accuracy are
tan be distinguished:
given in annex D.
- horizontal spatial variability in soil water con-
tent during the field calibration;
8.4 The accuracy of the relative or differential water
content (i.e. the Change in water content with time)
- small fluctuations in the shape of the water
will always be better than that of absolute values, be-
content Profile during the field calibration,
Cause some systematic errors (e.g. in the positioning
due to non-stationary flow conditions (see
of the calibration curve) are eliminated. To calculate
also annex A).
the accuracy of the differential water content, the

Together, these influences determine the residual error sources listed in 8.1 a), b) and c) tan be taken as

Standard deviation of the regression curve, i.e. a starting Point for the analysis of the propagation of

the calibration curve (Standard error of the re- errors through the relevant equations (i.e. the calibra-

gression). tion curve and the equation for calculating the differ-
ential water content).
c) Inaccuracy in the depth of placement of the probe
with respect to the calibration depth, particularly
when steep water content gradients occur.
9 Test report

8.2 When large variations in the shape of the water The test report shall include the following information:

content Profile occur, e.g. as a result of strong wetting

or evaporation fronts, the calibration curves are less a) a reference to this International Standard;

reliable and accuracy decreases accordingly.
b) an accurate s ite desc ription of the sampling lo-
cati on and cha racteriza tion of the soil Profile;
a description of the procedure used to install
8.3 When field calibration and measurements are
access tubes;
carried out under the conditions mentioned in this In-
a reference to an accurate description of the ap-
ternational Standard, the accuracy of the calculated
paratus used, with all necessary Performance
water content will also be determined by the number
characteristics;
of counts taken for each measurement [see 8.1 a)],
the number of samples for gravimetric determination
‘1 data on the calibration curves used;
that are taken for each soil layer and/or sampling lo-
the water content for various depths, in cubic
cation [see 8.1 b)] and the number and range of different
metres of water per cubic metre of soil;
hydrological conditions sampled. For sandy soil pro-

files of reasonable spatial homogeneity, an accuracy g) all observations that are important to the interpre-

of 0,005 m3/m3 to 0,Oi m3/m3, or 0,5 % (V/V) to tation of the results, such as the hydrological and

i,O % (V/V) in the calculated individual water content meteorological conditions before and during the

tan be reached, with moderate effort (see [Zl in an- measurements.
---------------------- Page: 7 ----------------------
ISO 10573:1995(E)
Annex A
(informative)
probe
Background information for the calii
Category 1) refers to water that does not evapor-
A.1 Introduetion
ate when soil is dried according to the prescribed
procedure (see ISO 11461).
This annex elaborates upon the theoretical Problems
associated with neutron probe calibration under prac-
Categories 1) and 2) include
tical circumstances.
- water present in confined pores;
- intercrystalline water, such as water
between clay plates;
A.2 Fundamental influenees on
measurements made with a neutron
- intracrystalline water, i.e. water of crystalliz-
depth probe ation;
- hydrogen present in aluminium hydroxides
Several factors influence the count rate measured at a
(bauxite laterite soils) or in organic com-
given soil water content. Distinction tan be made
pounds (peat soils).
between so-called homogeneous effects and non-
homogeneous effects. The first group refers to ef-
In all cases, the presence sf hydrogen in such
fects that are present when taking measurements in a
compounds may have a significant effect on the
homogeneous medium, i.e. in which the (Chemical)
thermalization process.
soil composition as well as the water content are uni-
form. The second group refers specifically to the ef-
Absorption of thermal neutrons by nuclei bvith a
fects caused by gradients in these Parameters within
large Cross-sectional area of absorption. The most
the measuring volume.
important elements in the context of soils are
boron, chlorine, iron and nitrogen because they
occur in abundante in certain situations.
A.2.1 Homogeneous effects
I ne factors mentionned under categories a) and b) in-
When measurements are carried out with a neutron
crease the measured count rate for a given water con-
depth probe in a homogeneous medium, the count
tent. Absorption of thermal neutrons [category c)],
rate at a given (free) water content is influenced by
however, decreases the saunt rate. The influence of
the following processes.
all these factors tan vary with time because of chan-
ges in the concentration of the compounds involved.
a) Thermalization due to collisions with atomic nuclei
This applies particularly to organic matter (Oxidation),
other than hydrogen nuclei in the soil measuring
iron and other metals and minerals (leaching influ-
volume.
enced by soils genesis), chlorine (in the case of Saline
soils) and nitrogen (fertilization and leaching).
Because they are such significant components of
soils, Oxygen and Silicon nuclei are the most
Changes in soil bulk density due, for example, to culti-
important. However, whereas an average of
vation, alter the concentration of all the compounds
17 collisions with a hydrogen nucleus are necess-
present in the soil and so modify the effects of the
ary to bring a neutron with an initial energy of
factors mentioned under a), b) and c).
1 MeV to a thermal energy level of 1/40 eV, this
requires 136 collisions with an Oxygen nucleus
and 240 collisions with a Silicon nucleus (see [IO]
A.2.2 Non-homogeneous effects
in annex E). The hydrogen present therefore do-
minates the thermalization process.
Non-homogeneous effects arise when gradients in
soil composition and/or water content are present
b) Collisions with hydrogen nuclei present in
within the measuring volume. For a given water content
at a certain depth, the probe count rate reflects the in-
1) non-free water (H,O); or
tegrated water content distribution within the measur-

2) those present in other compounds containing ing volume. This is also influenced by the generally

hydrogen.
hell-shaped impulse-response function (i.e. sensitivity
---------------------- Page: 8 ----------------------
@ ISO ISO 10573:1995(E)

distribution) of the detector. For the same water con- variables, h and d. Therefore, for a given water con-

tent at that depth, but with a different water content tent at a certain depth and a given combination of

state variables, different local water content distri-
distribution around it, the probe will give another re-
butions tan occur within the measuring volume. Non-
sult. Thus, for reproducible measurements, the water
stationary flow conditions occur mostly after severe
content distribution for a given water content at a cer-
rainstorms (wetting fronts) or after any other sudden
tain depth should be time-invariant. This condition tan
Change in hydrological conditions, hence also after the
be regarded as the basic requirement *for the field cali-
onset of a period of severe drought (evaporation
bration of the neutron depth probe.
front&
Another factor is the non-symmetric averaging within

the measuring volume, because the radius of the In practice, there is a fairly consistent seasonal course

measuring volume depends on the total atomic nuclei to the hydrological changes in the unsaturated Zone,

density [*l. This results in a net underestimation of the correspondig to a sequence of fixed combinations of h

average water content within the measuring volume and d. At any one time, this combination will vary

when a gradient of water content is present, irrespec- around the average combination. This results in slight

divergentes in water content measurements. The ef-
tive of its direction. This effect is also referred to as

the interface effect in the Iiterature. fett of hysteresis is similar. In field calibration, these

divergentes manifest themselves in spreading of cali-
In practice, the most severe examples of the interface
bration Points around the calibration curve and thus in
effect occur at the soil surface (soiI/air interface) and
the accuracy of this curve.
commonly also in the interface present between a

humus-rich topsoil and the subsoil or bedrock. For sites with a shallow water table, the following

applies.
Satisfying the requirement of time-invariant (local)
A.3 Hydrological state of the soil water
gradients is only possible when the state variables tan
be determined, and when stationary flow is taking
Time-invariant gradients of water content occur under
place. In practice, the depth of the phreatic level is the
certain hydrological conditions. At any Point in time,
more sensitive of the state variables. The reason for
the vertical distribution of water content is governed
this is the steep gradient d8/dh of the water retention
by the type of flow occurring in the unsaturated Zone.
curve in that area (i.e. h is approximately Zero), from

In soils with shallow water tab ’s, two types of flow which it follows that the gradient in the water content

tan be distinguished. is large at high water Potentials hydrostatic pressure
distribution (slightly negative to Zero), and small at low
water Potentials (large negative values). So, for a
a) Stationary flow (equilibrium conditions)
given depth of the phreatic level, the biggest Change

This is characterized by a constant vertical distribution in the water content Profile due to changing con-

of water content (steady - state water content ditions in the topsoil, will occur near the phreatic Ievel.

Profile), for given conditions in the topsoil (pressure But, under such wet conditions, the radius of the

head h) and a given depth d of the phreatic level
measuring volume, and thus the interface effect, is

(groundwater level), the so-called state variables. This minimal. Inversely, the radius of the measured volume

results in spatial-(depth-)invariant and time-invariant will be larger near the topsoil, but the gradient in the

capillary flux. Esch time that this combination of state water content Profile will be smaller. Hence the con-

variables occurs, the same local gra
...

SLOVENSKI STANDARD
SIST ISO 10573:2006
01-september-2006
.DNRYRVWWDO±'RORþHYDQMHYRGHYQHQDVLþHQLFRQL±0HWRGD]XSRUDER
QHYWURQVNHVRQGH

Soil quality -- Determination of water content in the unsaturated zone -- Neutron depth

probe method

Qualité du sol -- Détermination de la teneur en eau de la zone non saturée -- Méthode à

la sonde à neutrons de profondeur
Ta slovenski standard je istoveten z: ISO 10573:1995
ICS:
13.080.40 Hidrološke lastnosti tal Hydrological properties of
soils
SIST ISO 10573:2006 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

---------------------- Page: 1 ----------------------
SIST ISO 10573:2006
---------------------- Page: 2 ----------------------
SIST ISO 10573:2006
INTERNATIONAL
STANDARD
First edition
1995-12-15
Soil quality - Determination of water
content in the unsaturated zone - Neutron
depth probe method
Qualit& du so/ - Determination de Ia teneur en eau de Ia zone non saturke
- Methode a Ia Sonde ;j neutrons de profondeur
Reference number
ISO 10573: 1995(E)
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SIST ISO 10573:2006
ISO 10573:1995(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide
federation of national Standards bodies (ISO member bodies). The work of
preparing International Standards is normally carried out through ISO
technical committees. Esch member body interested in a subject for
which a technical committee has been established has the right to be
represented on that committee. International organizations, governmental
and non-governmental, in liaison with ISO, also take part in the work. ISO
collaborates closely with the International Electrotechnical Commission
(IEC) on all matters of electrotechnical standardization.
Draft International Standards adopted by the technical committees are
circulated to the member bodies for voting. Publication as an International
Standard requires approval by at least 75 % of the member bodies casting
a vote.
International Standard ISO 10573 was prepared by Technical Committee
lSO/TC 190, Seil quality, Subcommittee SC 5, Physical methods.
Annexes A, B, C, D and E of this International Standard are for information
only.
0 ISO 1995

All rights reserved. Unless otherwise specified, no par-t of this publication may be

reproduced or utilized in any form or by any means, electronie or mechanical, including

photocopying and microfilm, without Permission in writing from the publisher.
International Organization for Standardization
Case Postale 56 l CH-l 211 Geneve 20 l Switzerland
Printed in Switzerland
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SIST ISO 10573:2006
ISO 10573:1995(E)
INTERNATIONAL STANDARD @ ISO
Soil quality - Determination of water content in the
unsaturated Zone - Neutron depth probe method
- Neutron depth probes contain radioactive sources which will present health and
WARNING

environmental hazards if a probe is improperly used, stored or disposed of. National and international

legislation and regulations must be complied with.
vestigate the possibility of applying the most recent
1 Scope
editions of the Standards indicated below. Members
of IEC and ISO maintain registers of currently valid
This International Standard specifies an in situ method
International Standards.
for the determination of water content in the unsatu-
rated zone of soils using a neutron depth probe. lt is
ISO 11272: -l), Seil quality - Determination of dry
applicable when investigations into the water storage,
bulk density.
water balance and water distribution in the unsatu-
rated zone of the soil are carried out. Because the
ISO 11461: -l) Seil quality -
Determination of soil
method is non-destructive, it is particularly suitable for
wa ter content wlculated on a volume basis - Gravi-
repeated measurements at fixed locations. Water
metric method.
content profiles tan be obtained by measuring at a
series of depths down to any depth within the range
of the phreatic level at the site.
3 Definitions
The advantage of the method compared with some
others, for example the gamma probe method, is the
For the purposes of this International Standard, the
rapidity with which measurements tan be carried out.
following definition applies.
A disadvantage, however, is the relatively poor depth
resolution of the measurements.
3.1 water content volume fraction, 8 : The volume
of water evaporating from soil when dried to constant
mass at 105 OC, divided by the original bulk volume of
the soil.
2 Normative references
NOTES
The following Standards contain provisions which,
through reference in this text, constitute provisions of
1 The water content may be expressed as a percentage
this International Standard. At the time of publication,
by volume or a volume fraction.
the editions indicated were valid. All Standards are
subject to revision, and Parties to agreements based
2 In this International Standard, water content as defined

on this International Standard are encouraged to in- above may also be referred to as “free water ”.

1) To be published.
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SIST ISO 10573:2006
ISO ‘10573:1995(E) 0 ISO

3 The procedure for drying soil to constant mass at 105 “C - the gradients in this composition that occur within the

is described in ISO 11461. measuring volume;
- the gradients in soil water content that occur within
4 The procedure for determination of the bulk volume of
the measuring volume;
soil is described in ISO 11272.
- the method of access tube installation;
- the characteristics of the access tubing;
- the specifications sf the apparatus used.
4 Principle
The calibration curve usually differs for each soil layer. In
homogeneous layers that are thicker than the measuring
A neutron depth probe, consisting of a neutron Source
volume, calibration curves are generally linear, their para-
and detector, is lowered into a vertical access tube in
meters depending on the soil composition. In the case of
the soil. The neutron Source, usually of the 241Am-Be
thin or non-homogeneous soil Iayers, however, calibration
type, emits neutrons of high kinetic energy. The neu-
curves will often be non-linear due to the different effects
trons lose part of their energy when they collide with
of gradients in soil composition and water content under
atomic nuclei. After several collisions, their energy
wet and dry conditions.
level is reduced to the thermal energy level corres-
ponding to the prevailing temperature. This level is
reached most rapidly when neutrons collide with hydro-
gen nuclei because their masses are almost equal.
5 Apparatus
The thermal neutrons form a stable cloud, the concen-

tration of which is determined by the detector in the 5.1 Neutron depth probe, consisting of a fast neu-

probe. The number of thermal neutrons registered by tron Source and a thermal neutron detector combined

the detector per unit time (the count rate) is therefore with a read-out unit.
a measure of the concentration of hydrogen nuclei in
the soil around the probe. In general, the majority of
5.2 Thin-walled access tu iing, with an inner
those nuclei are in water molecules and therefore the
diameter slightly larger than that of the neutron probe.
count rate is also a measure of the soil water content.
The tubing shall consist sf rnaterial that is very
A calibration curve is used to convert the neutron
“transparent” to fast and thermal neutrons (e.g. alu-
count rate to soil water content.
minium, aluminium alloy) and which is resistant to
Chemical corrosion and to deformation due to instal-
NOTES
lation activities. Stainless steel, galvanized iron and
plastics (polyethylene) are also suitable, thouqh less
5 The neutron count rate obtained is influenced by the .
transparent to neutrons.
presence of all the atomic nuclei in the soil. However, the
count rate at a given water content may be increased in
some soils because of the thermalization of neutrons by
5.3 Equipment for instailing access tubes.
collisions with nuclei of certain soil elements, or because
much hydrogen is present in substances other than free
water. However, the count rate may be decreased because
ent for drying and cleaning the aceess
of absorption of neutrons by nuclei with a large atomic ab-

sorption Cross-section. See annex A. tubes, if necessary, a dummy probe for testing the

tubing Performance.
6 The soil volume (measuring volume) to which the measure-
ment refers approximates a sphere. For a given type of

neutron probe, the radius of the sphere depends on the to- curwes, for conversion of count rate

tal density of atomic nuclei in the soil. For the majority of
probes used in practice, the radius of the volume from
which 95 % of the neutrons counted by the detector are

generated ( “the sphere of importante” 111) tan vary from 5.6 Usual apparatus for takin so%8 samples, for

0,l m to 0,2 m in wet soil to 0,8 m or more in dry (sandy)
carrying out a field calibration to determine the volu-
soil. Consequently, the measurement obtained at a given
metric water content 8 gravimetrically according to
depth is influenced by the water content distribution within
ISO 11461.
the measuring volume at that time, and by any other gra-
dients in soil composition. Therefore, reproduction of the
measurement of a given water content at a certain depth is
only possible when the distributions of water content and
of soil composition within the measuring volume are time-
invariant. This requirement (local time-invariant gradients) is
important for the calibration of the neutron depth probe.
6.1 Installation of access tubes
See annex A.
lters of the calibration curve de-
7 The shape and parame
The location shall be representative of the immediate
Ilowing (see [2] in a nnex E):
pend on the fo
surroundings and care shall be taken to avoid surface
water from concentrating on the Spot. Use a platform
of the soil horizon considered
- the Chemical composition
to prevent darnage to surrounding Vegetation and
and its bulk d ensity;
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SIST ISO 10573:2006
0 ISO
ISO 10573:1995(E)
each soil layer in accordance with ISO 11461, under
compaction of the soil surface whilst installing a tube.

Ensure that radial soil compaction around the tube, several different hydrological conditions, to derive a

compaction below it and the creation of voids adjacent calibration curve for each layer.

to it are prevented as far as possible.
NOTE IO The subdivision of the soil Profile into layers is
determined initially by differentes in soil composition, but
Install access tubes by either of the following methods.
the form of soil water content gradients that systemati-
cally recur should also be considered. Further divisions may
a) Push the tube into the soil using a hammer and
be necessary to meet the objectives of the investigation.
empty it using an auger. lt is recommended that
the lower end of the tube be closed with quick
The hydrological conditions under which the calibra-
drying cement or a stopper, to prevent infiltration
tion is conducted shall differ as much as possible so
of ground water.
that the calibration curves are representative of the
range of conditions which occur at the site. To meet
b) Push the tube into a prepared hole of the same or
the requirement for time invariant gradients that do
slightly smaller diameter and of the required
not vary with time as much as possible, the calibration
depth, then seal the lower end as in 6.1 .l . Alterna-
shall not be conducted after heavy rain or irrigation
tively, the lower end of the tube may be sealed
applications, or immediately after the sudden begin-
before insertion.
ning of extremely warm weather.
Holes tan be prepared using a guide tube or an auger
Determine the calibration curves by analysing the va-
or by a combination of these two methods. Close the
rious combinations of neutron count rate and water
top of the tube with a tight rubber stopper to keep out
content for each soil layer by regression analysis. The
rain or surface water. The tubing shall always be dry
count rate is considered as the independent variable
inside.
(x) and the water content as the dependent variable
(y). Calibration curves so derived are specific to the
NOTES
neutron probe used. Use of reference counts to nor-
malize the count rate measurements used in the re-
8 lt is recommended that access tubes be tut to protrude
gression allows calibrations to be used with different
above the soil surface as little as the apparatus permits, so
probes of the Same geometry (see annex C).
as to minimize the radiation dose received by the Operator
when lowering the probe.
Further guidelines for carrying out a field calibration

9 More specific guidelines for installation are given in [3] are given in [Zl, [3], [4] in annex E and in annex B.

and [4] in annex E.
NOTES
After installation, take great care to minimize distur-
11 The calibration CU rves may Change in
time due to the
bance of the soil and Vegetation at the site whilst
foll owing processes:
conducting measurements in the access tube.
- changes in the Chemical composition of the soil including
that of the soil water, and changes in bulk density. This
tan be corrected for, to a certain extent, on the basis
6.2 Calibration
of known (Chemical) properties (see [3] in annex E);
- decrease of the Source strength of the probe due to ra-
In most cases, calibration curves supplied by neutron
dioactive decay, and/or decrease in the sensitivity of
probe manufacturers, and those published in the Iitera-
the detector. This tan be corrected for by the use of
ture, give only a rough indication of the absolute soil
reference counts made in a medium with invariant
water content, because no or insufficient recognition
characteristics (see annex C).
tan be given to the specific influences of the site

mentioned in note 7 in clause 4 (see also annex A). 12 The guidelines given here apply to the measurement of

absolute water content. When only relative measurements
(i.e. changes of water content in time) are to be assessed,
The influence of Chemical composition and bulk den-
the requirements for calibration and demands on accuracy
sity (see A.2) is accounted for in calibrations derived
may be less stringent.
theoretically from the macroscopic neutron-interaction
Cross-section of the soil concerned (see [ 11, [4], [9] in
annex E).
The combined influence of gradients in water content,
6.3 Measurements
Chemical composition and bulk density is only accounted

for by a field calibration. Therefore an in situ field cali- The neutron depth probe shall be used in accordance

bration is necessary for accurate measurements of with the manufacturer ’s instructions as much as poss-

absolute water content. ible, and particularly with respect to technical handling

and safety.
The field calibration is based on simultaneous deter-
Lower the probe in the ac cess-tube to the depth at
mination of the neutron count rate and sampling for
which It IS required to ma ke the meas uremen t.
the determination of the volumetric water content of
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SIST ISO 10573:2006
ISO 10573:1995(E) 0 ISO

Conduct the counts according to one of the following ed by appropriately skilled persons. Periodic Checks to

methods: test for leakage from the sealed Source shall be car-
ried out by a competent agency.
with a fixed counting ti me; in this case the num-
ber of thermal neutrons detected is reco rded;
with a fixed number of detected thermal neu-
trons; in this case the counting time is recorded. 7 Expressisn of results
NOTES
Calculate the count rate R, which is the number of de-
tected thermal neutrons per unit of time, using the
13 When changes of watet- content in time are to be de-
following equation:
termined, precise positioning of the probe at a specified
depth is important.
R N
14 The second method mentioned for taking the counts has
the advantage that the accuracy of the measurement is rela-
where
tively constant (i.e. precision of the count rate), whereas the ac-
curacy depends on the water content in the first method.
R is the count rate, in counts per minute;
N is the number of counted thermal neutrons;
Instead of conducting a Single count for a long time, it
tan be advantageous to make a number of counts for
t is the counting time, in minutes.
a short time because this provides quantitative infor-
mation about the spread of the measurements. This
Calculate the water content 8, using the equation:
information allows detection of certain types sf failure
in the apparatus.
0 = f fR9 PJ
lt is recommended that reference counts in a medium
where
with invariant characteristics, such as a large water
barrel (see C.3.1), be made at frequent intervals to
0 is the water content, expressed as a volume
check the Overall Performance of the instrument. For
fraction;
example, a reference count might be carried out
is the calibration function (calibration curve)
before and after each series of measurements in a
calculated by regression analysis;
specific access tube. A certain amount of drift in the

reference count is to be expected. However, a sudden R is the count rate, in counts per minute;

Change from the general Pattern almost certainly indi-
represents the Parameters of the calibration
cates a failure of the apparatus, which should be re-
curve.
paired or replaced.
When necessary, the count rate tan be corrected for
the differente between the actual reference count
6.4 Safety and maintenance
rate (R,) and the expected reference count rate IR,,).
In most cases, a correction of the type R’ = R(R,,IR,)
SAFETY PRECAUTIONS - The radioactive Source
may apply, where R’ is the corrected count rate. For
within a neutron depth probe is a potential hazard
further explanations, see annex C.
to the Operator, the public and the environment.
Most governments and Organkations have legally
enforceable regulations concerning the acqui-
sition, Operation, transport, storage and disposal
8 Accuracy
of radioactive devices, which must be adhered to.
In the absence of specific radiological safety regu-
.I The accuracy of the water content determined
lations, the guidelines of the International Atomic
with the neutron probe is influenced principally by the
Energy Agency Kl, r7] and of the International
following error sources.
Commission on Radiological Protection [*l should
be consulted.
a) The scatter in individual counts or count times as
a result of the random Variation in the number of
The half-life (458 years) of the americium commonly
neutrons emitted by the neutron Source.
used in neutron depth probes is longer than the time
over which the integrity of the Source Container (e.g.
The magnitude of this error is usually expressed
about 30 years) tan be expected to last. When a neu-
as the Standard deviation of the number of neu-
tron depth probe is no longer required, the radioactive
trons counted. As the emission process follows a
Source must be disposed of at a repository for radio-
Poisson distribution, the resulting Standard devi-
active waste.
ation in the number of detected neutrons is
Neutro n depth probes shall only be used by suitably
SN = JN
trained 0 #perato rs. Main tenance s hall only be conduct-
---------------------- Page: 8 ----------------------
SIST ISO 10573:2006
@ ISO ISO 10573:1995(E)
nex E). For soils that are more spatially variable with
The inaccuracy of the calibration curve used.
respect to water content (particularly clay, alluvium
This tan be determined from the results of the
and peat soils), a greater effort is necessary to resch
regression analysis used to derive the curve. Within
that accuracy. Further details with respect to conduc-
the field calibration, the following sources of errors
ting measurements and determination of accuracy are
tan be distinguished:
given in annex D.
- horizontal spatial variability in soil water con-
tent during the field calibration;
8.4 The accuracy of the relative or differential water
content (i.e. the Change in water content with time)
- small fluctuations in the shape of the water
will always be better than that of absolute values, be-
content Profile during the field calibration,
Cause some systematic errors (e.g. in the positioning
due to non-stationary flow conditions (see
of the calibration curve) are eliminated. To calculate
also annex A).
the accuracy of the differential water content, the

Together, these influences determine the residual error sources listed in 8.1 a), b) and c) tan be taken as

Standard deviation of the regression curve, i.e. a starting Point for the analysis of the propagation of

the calibration curve (Standard error of the re- errors through the relevant equations (i.e. the calibra-

gression). tion curve and the equation for calculating the differ-
ential water content).
c) Inaccuracy in the depth of placement of the probe
with respect to the calibration depth, particularly
when steep water content gradients occur.
9 Test report

8.2 When large variations in the shape of the water The test report shall include the following information:

content Profile occur, e.g. as a result of strong wetting

or evaporation fronts, the calibration curves are less a) a reference to this International Standard;

reliable and accuracy decreases accordingly.
b) an accurate s ite desc ription of the sampling lo-
cati on and cha racteriza tion of the soil Profile;
a description of the procedure used to install
8.3 When field calibration and measurements are
access tubes;
carried out under the conditions mentioned in this In-
a reference to an accurate description of the ap-
ternational Standard, the accuracy of the calculated
paratus used, with all necessary Performance
water content will also be determined by the number
characteristics;
of counts taken for each measurement [see 8.1 a)],
the number of samples for gravimetric determination
‘1 data on the calibration curves used;
that are taken for each soil layer and/or sampling lo-
the water content for various depths, in cubic
cation [see 8.1 b)] and the number and range of different
metres of water per cubic metre of soil;
hydrological conditions sampled. For sandy soil pro-

files of reasonable spatial homogeneity, an accuracy g) all observations that are important to the interpre-

of 0,005 m3/m3 to 0,Oi m3/m3, or 0,5 % (V/V) to tation of the results, such as the hydrological and

i,O % (V/V) in the calculated individual water content meteorological conditions before and during the

tan be reached, with moderate effort (see [Zl in an- measurements.
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SIST ISO 10573:2006
ISO 10573:1995(E)
Annex A
(informative)
probe
Background information for the calii
Category 1) refers to water that does not evapor-
A.1 Introduetion
ate when soil is dried according to the prescribed
procedure (see ISO 11461).
This annex elaborates upon the theoretical Problems
associated with neutron probe calibration under prac-
Categories 1) and 2) include
tical circumstances.
- water present in confined pores;
- intercrystalline water, such as water
between clay plates;
A.2 Fundamental influenees on
measurements made with a neutron
- intracrystalline water, i.e. water of crystalliz-
depth probe ation;
- hydrogen present in aluminium hydroxides
Several factors influence the count rate measured at a
(bauxite laterite soils) or in organic com-
given soil water content. Distinction tan be made
pounds (peat soils).
between so-called homogeneous effects and non-
homogeneous effects. The first group refers to ef-
In all cases, the presence sf hydrogen in such
fects that are present when taking measurements in a
compounds may have a significant effect on the
homogeneous medium, i.e. in which the (Chemical)
thermalization process.
soil composition as well as the water content are uni-
form. The second group refers specifically to the ef-
Absorption of thermal neutrons by nuclei bvith a
fects caused by gradients in these Parameters within
large Cross-sectional area of absorption. The most
the measuring volume.
important elements in the context of soils are
boron, chlorine, iron and nitrogen because they
occur in abundante in certain situations.
A.2.1 Homogeneous effects
I ne factors mentionned under categories a) and b) in-
When measurements are carried out with a neutron
crease the measured count rate for a given water con-
depth probe in a homogeneous medium, the count
tent. Absorption of thermal neutrons [category c)],
rate at a given (free) water content is influenced by
however, decreases the saunt rate. The influence of
the following processes.
all these factors tan vary with time because of chan-
ges in the concentration of the compounds involved.
a) Thermalization due to collisions with atomic nuclei
This applies particularly to organic matter (Oxidation),
other than hydrogen nuclei in the soil measuring
iron and other metals and minerals (leaching influ-
volume.
enced by soils genesis), chlorine (in the case of Saline
soils) and nitrogen (fertilization and leaching).
Because they are such significant components of
soils, Oxygen and Silicon nuclei are the most
Changes in soil bulk density due, for example, to culti-
important. However, whereas an average of
vation, alter the concentration of all the compounds
17 collisions with a hydrogen nucleus are necess-
present in the soil and so modify the effects of the
ary to bring a neutron with an initial energy of
factors mentioned under a), b) and c).
1 MeV to a thermal energy level of 1/40 eV, this
requires 136 collisions with an Oxygen nucleus
and 240 collisions with a Silicon nucleus (see [IO]
A.2.2 Non-homogeneous effects
in annex E). The hydrogen present therefore do-
minates the thermalization process.
Non-homogeneous effects arise when gradients in
soil composition and/or water content are present
b) Collisions with hydrogen nuclei present in
within the measuring volume. For a given water content
at a certain depth, the probe count rate reflects the in-
1) non-free water (H,O); or
tegrated water content distribution within the measur-

2) those present in other compounds containing ing volume. This is also influenced by the generally

hydrogen.
hell-shaped impulse-response function (i.e. sensitivity
---------------------- Page: 10 ----------------------
SIST ISO 10573:2006
@ ISO ISO 10573:1995(E)

distribution) of the detector. For the same water con- variables, h and d. Therefore, for a given water con-

tent at that depth, but with a different water content tent at a certain depth and a given combination of

state variables, different local water content distri-
distribution around it, the probe will give another re-
butions tan occur within the measuring volume. Non-
sult. Thus, for reproducible measurements, the water
stationary flow conditions occur mostly after severe
content distribution for a given water content at a cer-
rainstorms (wetting fronts) or after any other sudden
tain depth should be time-invariant. This condition tan
Change in hydrological conditions, hence also after the
be regarded as the basic requirement *for the field cali-
onset of a period of severe drought (evaporation
bration of the neutron depth probe.
front&
Another factor is the non-symmetric averaging within

the measuring volume, because the radius of the In practice, there is a fairly consistent seasonal course

measuring volume depends on the total atomic nuclei to the hydrological changes in the unsaturated Zone,

density [*l. This results in a net underestimation of the correspondig to a sequence of fixed combinations of h

average water content within the measuring volume and d. At any one time, this combination will vary

when a gradient of water content is present, irrespec- around the average combination. This results in slight

divergentes in water content measurements. The ef-
tive of its direction. This effect is also referred to as

the interface effect in the Iiterature. fett of hysteresis is similar. In field calibration, these

divergentes manifest themselves in spreading of cali-
In practice, the most severe examples of the interface
bration Points around the calibration curve and thus in
effect occur at the soil surface (soiI/air interface) and
the accuracy of this curve.
commonly also in the interface present between a

humus-rich topsoil and the subsoil or bedrock. For sites with a shallow water table, the following

applies.
Satisfying the requirement of time-invariant (local)
A.3 Hydrological state of the soil water
gradients is only possible when the state variables tan
be determined, and when stationary flow is taking
Time-invariant gradients of water content occur under
place. In practice, the depth of the phreatic level is the
certain hydrological conditions. At any Point in time,
more sensitive of the state variables. The reason for
the vertical distribution of water content is governed
this is the steep gradient d8/dh of the water retention
by the type of flow occurring in the unsaturated Zone.
curve in that area (i.e. h is approximately Zero), from

In soils with shallow water tab ’s, two types of flow which it follows that the gradient in the water content

tan be distinguished. is large at high water Potentials hydrostatic pressure
distribution (slightly negative to Zero), and small
...

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